The present invention relates generally to radar receivers and specifically to the correction of attenuation-induced errors in a weather radar receiver.
A weather radar operates on the principle that electromagnetic radiation is scattered by water droplets present in a rainstorm and that the amount of scatter is proportional to the rain density and thus the intensity of the storm. Some of the scattered radiation is received by the radar receiver and is amplified, detected and processed therein. It is desirable that the output video signal from the radar receiver represent a measure of the intensity of the storm in terms of rain density.
The received signal has a power level which is dependent upon the equipment utilized, transmission path effects, and the target which is providing the radiation scattering. Because it is the target (rain density) which is of primary interest, it is desirable to correct for the effects of equipment and the transmission path factors to provide the most accurate output available within reasonable economic constraints. A particular problem is the attenuation of the transmitted and reflected radar signals due to the presence of rain in the transmission path.
A typical weather radar illuminates a storm with a pulse transmission from a narrow beam antenna. Assuming the storm is large enough or close enough, the storm will intercept the entire beam cross-section and the interaction region (the point at which we desire information on the rain density) will be a disk-shaped region with a thickness equal to one-half the speed of light c times the pulse duration.
It can be shown that the ratio of the full beam power, P.sub.i (R) incident on a region at range R to the power transmitted, P.sub.t, is equal to the path transmission coefficient .nu.(R) and is given by Equation 1, where .lambda.'(R) is the effective volumetric loss coefficient of the target at the range R. ##EQU1## In a pulsed system R is equal to the speed of light c times the pulse travel time t divided by 2 and thus Equation 1 becomes: ##EQU2##
It can be shown that the effective volumetric loss coefficient .lambda.' is given by Equation 3, where .sigma. is equal to the volumetric scattering coefficient and K.lambda. and K.sigma. are constants of proportionality relating .lambda. (volumetric loss or absorbtion coefficient) and .sigma. to rain density. ##EQU3## substituting (3) into (2) we have ##EQU4## an empirical constant
Due to attenuation of the signal normally caused along the transmission path (spreading loss), it is desirable to gradually increase the overall receiver gain as a function of time t such that received echo signals and their variation in strength are indications of the rain density at range R. In order to estimate the volumetric scattering coefficient .sigma., it can be shown that the receiver transfer characteristic G (gain) should vary as a function of time as shown in Equation 5, where A.sub.r is equal to the effective aperature of the receiving antenna, and .tau. is the transmitted pulse duration ##EQU5## where G.sub.o represents the constant component of receiver gain, dependent upon the particular radar system parameters, and .gamma.(t) is computed continuously as a function of the detected receiver output .sigma. and can be computed by the blocks shown in FIG. 1. If properly computed, .gamma.(t) in (5) will effectively compensate for attenuation of the signal caused by precipitation in the transmission path.